linux-kernel - Re: [PATCH] mm: proc: smaps_rollup: Fix pss

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Date:   Tue, 29 Jan 2019 16:52:21 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Sandeep Patil <sspatil@...roid.com>
Cc:     adobriyan@...il.com, avagin@...nvz.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        kernel-team@...roid.com, dancol@...gle.com
Subject: Re: [PATCH] mm: proc: smaps_rollup: Fix pss_locked calculation

On 1/29/19 1:15 AM, Andrew Morton wrote:
> On Sun, 20 Jan 2019 17:10:49 -0800 Sandeep Patil <sspatil@...roid.com> wrote:
> 
>> The 'pss_locked' field of smaps_rollup was being calculated incorrectly
>> as it accumulated the current pss everytime a locked VMA was found.
>> 
>> Fix that by making sure we record the current pss value before each VMA
>> is walked. So, we can only add the delta if the VMA was found to be
>> VM_LOCKED.
>> 
>> ...
>>
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -709,6 +709,7 @@ static void smap_gather_stats(struct vm_area_struct *vma,
>>  #endif
>>  		.mm = vma->vm_mm,
>>  	};
>> +	unsigned long pss;
>>  
>>  	smaps_walk.private = mss;
>>  
>> @@ -737,11 +738,12 @@ static void smap_gather_stats(struct vm_area_struct *vma,
>>  		}
>>  	}
>>  #endif
>> -
>> +	/* record current pss so we can calculate the delta after page walk */
>> +	pss = mss->pss;
>>  	/* mmap_sem is held in m_start */
>>  	walk_page_vma(vma, &smaps_walk);
>>  	if (vma->vm_flags & VM_LOCKED)
>> -		mss->pss_locked += mss->pss;
>> +		mss->pss_locked += mss->pss - pss;
>>  }
> 
> This seems to be a rather obscure way of accumulating
> mem_size_stats.pss_locked.  Wouldn't it make more sense to do this in
> smaps_account(), wherever we increment mem_size_stats.pss?
> 
> It would be a tiny bit less efficient but I think that the code cleanup
> justifies such a cost?

Yeah, Sandeep could you add 'bool locked' param to smaps_account() and check it
there? We probably don't need the whole vma param yet.

Thanks.